This invention relates generally to fiber optical radio-frequency communication or data links, and more particularly, to those communication applications of optical fibers in which a high dynamic range is required. Dynamic range is analogous to a signal-to-noise ratio. In a practical environment, every radio communication signal is distorted by noise components derived from a variety of sources, and by harmonic distortion components at frequencies that are integral multiples of the signal frequency. Dynamic range may be defined as the ratio of the available signal power to the total power associated with noise and harmonic distortion components. Thus, a larger value of dynamic range is equivalent to a purer or less distorted signal.
Relatively high values of dynamic range are obtainable using conventional coaxial cable as a communication link, and transmitting the signal at radio frequencies. Typically, dynamic range values in excess of approximately 83 decibels (dB) are attainable in coaxial cables. However, there are some situations in which coaxial cables are unsuitable for various reasons. For example, if the required transmission distance of the link is too great, the advantage of coaxial cables is diminished by their high transmission losses, in comparison to low-loss optical fibers. Another example is an application that requires repeated flexing of the communication link, as in rotatable antennas. In this case, there may be mechanical problems, such as excessive mass, fatigue failure, and so forth, that dictate the use of optical fibers instead of coaxial cables. Susceptibility to electromagnetic interference (EMI) is yet another factor that may dictate the use of optical fibers instead of coaxial cables.
Use of a fiber optical link avoids many of these problems. Fibers are lighter in weight and immune to electromagnetic interference. They are also relatively resistant to fatigue failure due to repeated flexing. In a fiber optical communication link, a carrier signal at or near visible light frequencies is employed. In most applications of interest, the communication signal is available in the form of a modulated radio-frequency (rf) signal. For example, an rf signal may be received at an antenna and the application calls for transmitting it over optical fibers to a distant location. Accordingly, such applications require that an optical frequency carrier signal be modulated with an rf subcarrier signal, which in turn carries the communication signal. This invention is not concerned with the method of rf modulation that is used, except to the extent that some rf modulation techniques require a greater dynamic range than others.
The conventional technique for modulating an optical carrier signal source, such as an injection laser diode, is to bias the laser to a point in its operating characteristic above its lasing threshold, and to apply simultaneously a modulating (rf) signal. However, the resulting modulated signal includes a strong second harmonic component, as well as higher-order harmonics. The dynamic range is, therefore, relatively poor, only about 33 dB, compared with 83 dB for a typical coaxial cable.
It will be appreciated from the foregoing that there is a need for an alternative approach for the modulation of an optical beam in a fiber optical communication link. The present invention is directed to this end.